What's the difference between process models and approaches. In Ian Sommerville's Software Engineering book, Software processes categorized in to Plan-Driven and Agile. Then he talks about 3 models, Waterfall, Incremental and Reuse-oriented. Then he talk about Rational unified process and then it comes a topic about Boehm spiral model. Is this Boehm spiral model a another type of software process model as Waterfall, Incremental and Reuse-oriented?
Then in Agile software development, he's talking about RAD (Rapid Application Development) is this also a process model? Can we categorize software process models in to the 2 processes (i.e Plan driven and agile)?
A process or method, in this context, is a set of steps that produce a software product.
A process model is a generic description of a category of processes.
So, waterfall, incremental, and reuse-oriented are all categories of processes. There are potentially several processes that fall into each category.
A spiral model is another category, which is generally concerned with managing certain developmental risks. Barry Boehm was the first author who described a spiral model (and he described it as a "process model") and his description is labelled as Boehm's Spiral Model by subsequent authors.
In practice, there is nothing stopping someone from developing a process that draws features from a number of process models. It is then a bit of a philosophical (sometimes almost religious) discussion as to whether such a process fits into multiple categories, or in its own category.
RAD is also described as a process model, although the term is sometimes (e.g. by James Martin) applied to particular processes. So, depending on who you ask, RAD is either a particular development process or a category of development process.
Some authors do use the terms "process" and "process model" interchangeably (RAD being either a process or a process model, depending on who you ask, is an example) which muddies the water even further.